On Windows 7 professional, I want a bigger icon just for one of the shortcuts on the desktop (only one program shortcut should have large icon, rest of the shortcut icons should be of the original/default size), is there a workaround like putting a "clickable" jpg onto the desktop?
No, icons on the desktop are all the same size and by default fit into a grid. You cannot have an icon with a different size.
What you could do is writing a little program that basically appears to be an icon and just let it never come on top of other windows. You could then proceed to do whatever you want if it's clicked.